The present invention relates to a displacement measuring device that includes a gear mounted to shaft and engaged with a rack. The gear is pushed toward the rack by a spring so that the displacement of the diameter of the revolutions of the shaft can be precisely measured.
A conventional displacement measuring device for measuring the sum of displacement of the diameter of a shaft is shown in FIG. 1 and generally includes a body 10 which is moved transversely relative to rails and a shaft 11 is rotatably connected to the body 10 and driven by a driving means which is not shown in the drawings. A gear 14 is mounted to the shaft 11 and a rack 16 is engaged with the gear 14. When the body 10 is moved, the gear 14 engaged with the rack 16 rotates and the number of the revolution of the gear 14 is counted by a counting member 13 connected to an end of the shaft 11. The sum of the displacement of the object 10 can be measured by calculating the number of revolution of the shaft 11 times the diameter of the shaft 11. As shown in FIG. 2, there is a gap xe2x80x9cDxe2x80x9d between the tooth of the rack 16 that is engaged with the tooth of the gear 14. In other words, a tolerance is generated when a tooth of the gear 14 engages with the tooth of the rack 16 so that the sum of the displacement of the object 10 is not precise enough.
The present invention intends to provide a displacement measuring device wherein a spring pushes the gear toward the rack so that the gap xe2x80x9cDxe2x80x9d is reduced and the tolerance is reduced.
In accordance with one aspect of the present invention, there is provided a displacement measuring device which comprises a body which is movable on a plurality of rails and an extension portion is connected to the body and includes a spring received therein. A first shaft and a second shaft are connected to the body which is driven by the first shaft. A first gear and a second gear are respectively mounted to the first shaft and the second shaft. The second shaft extends through the extension portion and a counting member is connected to the second shaft. A rack is engaged with the first gear and the second gear. The spring pushes the second shaft so as to push the second gear toward the rack.